This invention relates to an object detecting switch device. More particularly, the invention relates to an object detecting switch device which makes use of a common transducer for transmission and reception of ultrasonic signals.
Conventionally, an ultrasonic switch device transmits and receives ultrasonic signals to and from an object to be detected. However, this kind of ultrasonic switch produces echoes which prevents the switch from detecting the object unless the object is located close enough to reflect the ultrasonic signals within the range of the echoes.
In order to detect a short-distant object having reflective ultrasonic signals within the range of the echoes, the distance of the object is calculated by measuring a time period between first and second receiving signals. In another manner, a detection signal is produced if a signal is received during the opening period of a gate.
As illustrated in FIGS. 5(A) and 5(B), immediately after an echo E, it is impossible to determine whether or not a receiving signal is a first receiving signal unless a second receiving signal arrives. In FIG. 5(A), an object is shown to be in a location of 12 cm., while in FIG. 5(B) the object is shown at a distance of 6 cm. away, which is understood by referring to the second and third receiving signals (b2 and b3), respectively.
For this purpose, however, a detecting gate needs to be kept open for a period corresponding to 40 cm. if a detecting switch is intended for detecting an object located at a distance of 20 cm. Thus, constant reference to the first and second signals takes time in terms of object detection, and further has the disadvantage of easily picking up noises.
On the other hand, if reference is made only to the second receiving signal in the detection of an object, signals b1 and b2 cannot be distinguished between a detection range of 8 cm. or 16 cm., as shown in FIGS. 6(A) and 6(B). Thus, it is difficult to determine the range of detection displayed on a switch. Further, in FIG. 6(A), in a case wherein a detection gate is set up, for example, between a range of 10 to 15 cm., the detectable ranges are 10 to 15 cm. and 5.0 to 7.5 cm., which results in an undetectable range of 7.5 to 10.0 cm. Consequently, a non-continuous range is detected.